Pulsed Porphyry Cu-Mo Formation during Protracted Pluton Emplacement in Southern Armenia, Lesser Caucasus: the Potential Role of Crustal Melting for Ore Recycling

Abstract

The southern part of the Lesser Caucasus records a long lasting geological and metallogenic evolution from the Jurassic to the Cenozoic. The composite Meghri-Ordubad pluton, which hosts porphyry Cu-Mo deposits, includes Eocene to Miocene intrusive rocks over an area of 1400 km2. Based on LA-ICP-MS UPb zircon dating of thirty samples, we document three main magmatic events: (1) mid-Eocene from 46 to 43 Ma, (2) Eocene-Oligocene from 38 to 31 Ma, and (3) Oligocene-early Miocene from 27 to 21 Ma. Based on Re-Os molybdenite dating, each magmatic event is associated with porphyry Cu-Mo (or Mo-Cu) formation, including the giant Kadjaran porphyry Mo-Cu deposit. We propose an evolution from subduction-related calcalkaline magmas to collisional alkaline magmas followed by bi-modal magmatism over time. Preliminary Nd and Sr isotopic compositions of the magmatic rocks broadly indicate an increasing mantle-derived component with younger magmatic events. Hf isotopic compositions of zircons range from +8 to +11 for Jurassic, Cretaceous, Eocene, Oligocene and Miocene rocks. Based on Hf isotopic composition mixing models involving a depleted mantle and Jurassic to Miocene crustal rocks, we propose that an important volume of crustal rocks can be re-melted. We suggest an ore recycling process by crustal melting leading to the metal enrichment of younger magmas, especially during ore-forming protracted pluton emplacement